What is A-Weighted Decibels (dBA)?
A-weighted decibels (dBA) are a measurement scale that adjusts sound levels to match how the human ear perceives different frequencies. Unlike standard decibels (dB) that measure all sound equally, dBA filters out low and very high frequencies that human hearing is less sensitive to.
Key Point: Noise ordinances almost always use dBA measurements because they reflect what people actually hear, making them legally relevant for complaints.
The Science Behind A-Weighting
The human ear doesn't hear all frequencies equally. We're most sensitive to sounds between 2,000-4,000 Hz (speech range) and less sensitive to very low (below 500 Hz) and very high (above 10,000 Hz) frequencies. A-weighting applies a frequency filter that reduces the contribution of low and high frequencies to the overall decibel reading.
Typical A-Weighting Effects:
- • Low frequencies (20-100 Hz): Reduced by 20-50 dB
- • Speech frequencies (1,000-4,000 Hz): Minimal reduction
- • High frequencies (10,000+ Hz): Reduced by 10-20 dB
Why dBA Matters for Noise Complaints
When documenting noise for legal evidence, dBA measurements provide the most accurate representation of the disturbance's impact on people. For example, a construction site might produce 90 dB of low-frequency noise, but if the human ear can barely hear those frequencies, the dBA reading might only be 65 dB—closer to the actual annoyance level experienced by residents.
Real-World Example
A neighbor's bass-heavy stereo might register 85 dB on an unweighted meter, but only 65 dBA when A-weighted. This 20 dB difference significantly affects whether the noise violates local ordinances, which are written in dBA for precisely this reason.
Important Considerations
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Always verify whether local noise ordinances specify dBA or unweighted dB measurements—most use dBA.
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A-weighting is standardized by IEC 61672, ensuring consistent measurements across different equipment.
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Some specialized noise sources (like industrial machinery) may require additional frequency analysis beyond dBA.